Galactosemia is a condition where the sugars in the body can not be broken down. Specifically, the body can not break down galactose. Because Lactose is a larger molecule that contains galactose, it can not be broken down either. When the sugar can't be broken down, it creates a buildup of different substances that can be toxic to the body.

Treatment, as you know is to eliminate lactose and galactose from the diet.